Many metallic components wear in use. Vanes of the compressor sections of aircraft engines, for example, may develop cracks or sustain other damage over time. Because the cost of replacing these vanes is relatively high, they often are reconditioned or otherwise repaired for further use. Those with severely damaged trailing edges typically have the injured portions removed and similar, undamaged pieces welded in place. The welded pieces thereafter may be ground, albeit laboriously, to match the shapes of those removed.
Alternatively, for some worn vanes a flat, pre-sintered boron-containing sheet with greater than 99% density may be cut to the required size, tack welded to the vane, and then brazed in place. It too must be ground to duplicate the original shape of the vane, although such grinding often is minimal. However, if substantial quantities of boron are used as a melting-point depressant, an unacceptable amount of boron may be transferred to the substrate.
Extrudable pastes additionally may be used in conjunction with "green" (i.e. not sintered or melted) metal-loaded sheets. Such pastes, containing the same metal powders as their associated sheets, are applied along the edges of the green sheets to encase areas no longer covered when the sheet shrinks during brazing. These pastes again often add undesirable quantities of boron to the vanes, however and, following brazing, require substantial grinding to achieve required shapes.
U.S. Pat. No. 3,743,556 to Breton, et al. discloses yet another process for coating a metallic substrate with a composition of a hard abrasive material dispersed in a hard metal matrix. According to the Breton patent (at column 1, lines 61-71), the invention disclosed therein
consists of laying on the substrate a film of a desired thickness of metal or alloy matrix material in an organic binder. A second film of powdered filler such as tungsten carbide in an organic binder is placed on the substrate contiguous to the filler filled film. The filler material is characterized as being wetted by the matrix metal or alloy in the molten state. This assembly is heated to decompose the binder and melt the matrix metal or alloy which is infused by capillary action into the filler layer. Cooling yields a coated substrate having a filler filled, void-free coating of the matrix material.
The film includes polytetrafluoroethylene (PTFE) and is rolled rather than cast. The Breton patent discloses as adhesives solely organic substances that decompose and evaporate at relatively low temperatures (e.g. 200.degree. C. for polymethacrylate), precluding adhesion of the film to the substrate when opposed by gravity during brazing, for example. It additionally neither teaches nor suggests avoidance of surface cracks caused by linear shrinkage.
U.S. Pat. No. 5,263,641 to Rafferty, et al. describes a brazing paste including an alloy powder, a binder, and a minor amount of finely-ground powered PTFE. Smearing the paste onto the substrate using a spatula or other device and then using it to bond a metal object to the substrate purportedly eliminates the need to coat the substrate with nickel to avoid formation of oxides. The Rafferty patent fails to disclose use of any dried materials such as tapes or films, however. As with the Breton patent, moreover, the Rafferty patent neither teaches nor suggests either using an inorganic adhesive prior to heating or any mechanism for avoiding surface cracks caused by shrinkage. This lack of disclosure of crack avoidance is consistent with the remainder of the Rafferty patent, which contemplates use of only (low melting point) alloys that melt completely during brazing.